It is no exaggeration to say that this King Lear is long-awaited. Critics, in fact, have been waiting impatiently for nine weeks to get a glimpse of a production that has been playing to a paying public. But, however absurd the delay, I can report that Ian McKellen is a majestic, moving Lear and that Trevor Nunn's production, while nothing like as radical as Brook's or Hytner's, is largely satisfying.

It begins thrillingly. We are in a late 19th century world evoked by Christopher Oram's design of a curving, elliptical, heavy-curtained theatrical balcony. To a thunderous peal of organ music, McKellen's gold-robed Lear enters between lines of courtiers to bestow new honours upon his beloved Cordelia. All this, in its reminiscence of Ivan the Terrible, is pure Eisenstein; and what it tells us, before a word has been spoken, is that Lear occupies a kingdom steeped in elaborate, meaningless ritual.

The idea is continued wittily in the first scene. The division of the realm is clearly a staged event in which McKellen reads a prepared speech, announcing that he will "unburdened crawl towards death" with a wry chuckle: obviously, he intends the exact opposite. Goneril and Regan also deliver their pronouncements of love from a lectern. But it is Cordelia who cuts through the pageantry with a refusal to play the game that unwittingly provokes the entire tragedy.

What follows is a stripping away, in every sense, of public ceremony to reach the square root of humanity. And, in McKellen's case, I was reminded of his early triumph as Richard II. That was a young king who was encased in ritual and who had to learn about human suffering: his Lear is an old king who has to undergo a similar moral journey in which he acquires sanity via madness. And what makes McKellen so moving is his awareness of the path he has to undertake. His simple statement of "I did her wrong" pricks one's tears because of Lear's premonition of the pain that is his due.

Some may be shocked that McKellen's remorseless journey to the centre of his self leads him, at one point, to strip to the buff. But the gesture seems entirely logical. McKellen has just been staring at an almost naked Poor Tom as if he were a scientific specimen before declaring: "Unaccommodated man is no more but such a poor, bare forked animal as thou art." This is Lear's moment of revelation in which he sees the truth as to what lies behind the carapace of costume. And, by his own disrobing, Lear acknowledges he is a member of the human race. Only those with dirty minds will be dismayed by McKellen's nudity.

But what is most striking about his Lear, in its pilgrim's progress, is its curiosity. McKellen's Lear is a man who is always asking questions. The big conundrum, which he delivers with racking slowness, is: "Is there any cause in nature that makes these hard hearts?" And it is his uncertainty as to the answer that touches one's own heart. But, even to the last, McKellen is a probing, questioning figure staring quizzically at Kent and asking: "Who are you?" By the end, you feel this is a Lear who has somehow undergone a rigorous moral education.

If I miss anything in Nunn's well-ordered production, it is the dizzying, vertiginous senselessness that pervaded Hytner's 1990 version with John Wood. The Courtyard, for all its supposed intimacy, also thrusts much of the action upstage so that the actors often seem distant even from the front stalls. But Frances Barber, whose knee accident prompted the first night's postponement, gives us a good, uncomplicated Goneril who is easily the harshest of the three sisters: one notices how she prevents Monica Dolan's Regan expressing her fellowship with the distressed Lear. Romola Garai also allows us to glimpse Cordelia's spontaneous virtue.

I felt neutral about Sylvester McCoy's Fool: a spoon-playing old joker in a tasselled toque who missed some of the character's piercing veracity. And Philip Winchester's Edmund could have used a bit more villainous buoyancy.

But there is solid support from William Gaunt's Gloucester, Jonathan Hyde's Kent and Julian Harries's Albany, who all demonstrate that goodness can exist, even in an anarchic world. And, if any theme emerges from Nunn's production, it is not just the vanity of ceremony but the need for human endurance. But it is for McKellen, and his triumphant progress towards a kind of enlightenment, that I shall really remember the occasion.

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Nudity, madness, Marx: A history of Lear

King Lear was first performed for King James I at Whitehall Palace on December 26 1606.

David Garrick played Lear, opposite Margaret Woffington as Cordelia, in May 1742.

John Gielgud first played Lear on stage in 1931, and many times after that. Historians of the Royal Shakespeare Company would record Gielgud's Lear as the jewel in the crown of its 1950 season.

Paul Scofield's King Lear ran and toured from 1962-64.

Director Peter Brook saw the role of Lear as "a mountain whose summit has never been reached, the way up strewn with the shattered bodies of earlier visitors - Olivier here, Laughton there: it's frightening".

In 1970 Soviet director Grigori Kozintsev's film gives the drama a Marxist conclusion in which the peasant victims of the royal saga start to clear the battlefield with a view to rebuilding the society.

Max Stafford-Clark's production at the Royal Court in 1993 focused on sexual politics, with Lear (Tom Wilkinson) reigning over a militaristic court and employing a drag Fool.

Ian Holm in Richard Eyre's 1997 production at the National's Cottesloe theatre appeared naked for part of the heath scene. The production was a hit with critics and audiences.